The plastic deformation simulation of AZ31 magnesium alloy at elevated temperature was performed on Gleeble-1500 hot simulator with strain rates of 0.01, 0.1, 1, 5 and 10 s-1 over a temperature range from 473 to 723 K and the maximum deformation degree of 80%. According to the obtained true stress-true strain curve, the relationship between flow stress and deformation temperature as well as strain rate were analyzed and the materials parameters and the activation energy were calculated. The constitutive relationship was established with Zener-Hollomon parameter and a comparison between the measured stresses and calculated stresses was made. The results show that the flow stress increases with the increase of strain rate at constant temperature, and decreases with the increase of deformation temperature at constant strain rate, and there are kinds of flow stress in different deformation models. The established constitutive relationship can reflect the change of flow stress during hot deformation.